1. Field of the Invention
The present invention relates to an immersion exposure apparatus, which projects a pattern of an original onto a substrate via a projection optical system and a liquid, thereby exposing the substrate, and a method of manufacturing a device using the immersion exposure apparatus.
2. Description of the Related Art
Along with the advance of the micropatterning of semiconductor devices, exposure light sources are shifting from high-pressure mercury lamps (for example, g-line and i-line) to ones that emit light beams having shorter wavelengths, such as a KrF excimer laser and an ArF excimer laser. To obtain a higher resolving power, it is necessary to increase the NA (numerical aperture) of the projection optical system, so that the depth of focus is further decreasing. The relationship among these factors can be generally expressed by:(resolving power)=k1(λ/NA)(depth of focus)=±k2(λ/NA2)where λ is the wavelength of the exposure light, NA is the numerical aperture of the projection lens, and k1 and k2 are coefficients that depend on the process involved.
As techniques for increasing the resolving power and the depth of focus, a phase shift mask and modified illumination have already been studied and put to practical use. As another technique for increasing the resolving power and the depth of focus, an immersion exposure technique is known. The immersion exposure technique performs exposure by filling the interval between the substrate surface (image plane) and the end face of the projection optical system with a liquid having a high refractive index.
The interval between the image plane and the end face of the projection is called a working distance. In a conventional dry exposure apparatus, the working distance is filled with, e.g., air. The working distance is usually set to 10 mm or more for convenience of accommodating an auto focus system in the apparatus. In the immersion exposure apparatus, however, the working distance is reduced to 3 mm or less. This increases the probability that the projection optical system and the substrate stage that holds the substrate will come into mechanical contact with each other, as compared with the dry exposure apparatus.
Still worse, when the liquid that fills the working distance leaks out due to some cause, this may exert an adverse influence on, e.g., electronic devices or mechanisms in the exposure apparatus. When the liquid leaks out or the projection optical system and the substrate stage come into mechanical contact with each other, huge amounts of time are required for maintenance until recovery, resulting in deterioration in productivity.
Especially, assume that electrical power supply to the substrate stage mechanism is shut off due to some cause during the operation of the exposure apparatus. In this case, for example, the liquid may remain in the working distance, the liquid may leak out thereafter, or the projection optical system and the substrate stage may come into mechanical contact with each other.
To solve these problems, the following techniques have been proposed as measures to prevent, when electrical power supply to the substrate stage mechanism is shut off due to some cause, such liquid leakage and mechanical contact between the projection optical system and the substrate stage. Japanese Patent Laid-Open No. 2005-079480 discloses a proposal of forming a liquid recovery groove on the substrate stage in advance, and tilting the substrate stage so that the liquid flows into the recovery groove if an emergency such as an electrical power failure occurs. Japanese Patent Laid-Open No. 2005-268742 discloses a proposal of stopping liquid supply if an emergency such as an electrical power failure occurs, and specifying a liquid leakage portion by a liquid leakage detection mechanism, thereby preventing the adverse influence of the liquid leakage on, e.g., other peripheral electronic devices. Japanese Patent Laid-Open No. 2006-179761 discloses a proposal of providing a plurality of power supply systems.
However, in the technique described in Japanese Patent Laid-Open No. 2005-079480, the substrate stage must be tilted very largely by taking account of the physical properties (e.g., the affinity) of the substrate and the liquid. In addition, the probability that the projection optical system and the substrate stage will come into mechanical contact with each other still remains. The technique described in Japanese Patent Laid-Open No. 2005-268742 is effective in reducing the adverse influence exerted by the liquid leakage, but cannot prevent the liquid leakage itself and mechanical contact between the projection optical system and the substrate stage. Japanese Patent Laid-Open No. 2006-179761 does not disclose how to control the substrate stage when electrical power supply from the main power supply is shut off.